Microsoft Reader is a desktop application for reading eBooks. It allows you to organize, annotate and bookmark your eBook collection. However, there are some major reservations!
While it has very readable text, and the annotation functions are really good it breaks down when you want to import eBooks. You can purchase eBooks from the Microsoft Reader store, but try to import your own eBooks and you come up against a brick wall. There is no toolbar, and no obvious way to import eBooks, and dragging and dropping doesn't work. Importing Word files also doesn't function.
This program hasn't been updated since 2005, so it seems as if Microsoft has given up on the program, and as it seems to be as good as broken, that's probably not a bad thing. It needs a proper interface, and more openness to the variety of eBook files around.
If you want an eBook reader for Windows, make sure it's not Microsoft Reader, it's a genuinely bad piece of software.